Hybrid coordination function (HCF) access through tiered contention and overlapped wireless cell mitigation

ABSTRACT

A method and system reduce interference between overlapping first and second wireless LAN cells in a medium. Each cell includes a respective plurality of member stations and there is at least one overlapped station occupying both cells. An inter-cell contention-free period value is assigned to a first access point station in the first cell, associated with an accessing order in the medium for member stations in the first and second cells. The access point transmits a beacon packet containing the inter-cell contention-free period value, which is intercepted at the overlapped station. The overlapped station forwards the inter-cell contention-free period value to member stations in the second cell. A second access point in the second cell can then delay transmissions by member stations in the second cell until after the inter-cell contention-free period expires. The beacon packet sent by the first access point station also includes an intra-cell contention-free period value, which causes the member stations in the first cell to delay accessing the medium until polled by the first access point. After the expiration of the intra-cell contention-free period, member stations in the first cell may contend for the medium based on the quality of service (QoS) data they are to transmit, using the Tiered Contention Multiple Access (TCMA) protocol.

[0001] This application claims the benefit of the following co-pendingapplications:

[0002] [1] U.S. Provisional Application Serial No. 60/302,661, filedJul. 5, 2001, entitled “HCF Access Through Tiered Contention”;

[0003] [2] U.S. Provisional Application Serial No. 60/304,122, filedJul. 11, 2001, entitled, “HCF Access Through Tiered Contention,” and

[0004] [3] U.S. Provisional Application Serial No. 60/317,933, filedSep. 10, 2001, entitled “HCF Access and Overlapped BSS Mitigation”; allof which are incorporated herein by reference.

RELATED APPLICATIONS

[0005] This patent application is related to the copending regularpatent application Ser. No. 09/985,257, filed Nov. 2, 2001 by MathildeBenveniste, entitled “Tiered Contention Multiple Access (TCMA): A Methodfor Priority-Based Shared Channel Access”, which is incorporated byreference.

FIELD OF THE INVENTION

[0006] The invention disclosed broadly relates to telecommunicationsmethods and more particularly relates to Quality of Service (QoS)management in multiple access packet networks.

BACKGROUND OF THE INVENTION

[0007] Wireless Local Area Networks (WLANS)

[0008] Wireless local area networks (WLANs) generally operate at peakspeeds of between 10 to 100 Mbps and have a typical range of 100 meters.Single cell Wireless LANs, are suitable for small single-floor officesor stores. A station in a wireless LAN can be a personal computer, a barcode scanner, or other mobile or stationary device that uses a wirelessnetwork interface card (NIC) to make the connection over the RF link toother stations in the network. The single-cell wireless LAN providesconnectivity within radio range between wireless stations. An accesspoint allows connections via the backbone network, to wirednetwork-based resources, such as servers. A single cell wireless LAN cantypically support up to 25 users and still keep network access delays atan acceptable level. Multiple cell wireless LANs provide greater rangethan does a single cell, by means of a set of access points and a wirednetwork backbone to interconnect a plurality of single cell LANs.Multiple cell wireless LANs can cover larger multiple-floor buildings. Amobile laptop computer or data collector with a wireless networkinterface card (NIC) can roam within the coverage area while maintaininga live connection to the backbone network.

[0009] Wireless LAN specifications and standards include the IEEE 802.11Wireless LAN Standard and the HIPERLAN Type 1 and Type 2 Standards. TheIEEE 802.11 Wireless LAN Standard is published in three parts as IEEE802.11-1999; IEEE 802.11a-1999; and IEEE 802.11b-1999, which areavailable from the IEEE, Inc. web sitehttp://grouper.ieee.org/groups/802/11. An overview of the HIPERLAN Type1 principles of operation is provided in the publication HIPERLAN Type 1Standard, ETSI ETS 300 652, WA2 December 1997. An overview of theHIPERLAN Type 2 principles of operation is provided in the BroadbandRadio Access Networks (BRAN), HIPERLAN Type 2; System Overview, ETSI TR101 683 VI.I.1(2000-02) and a more detailed specification of its networkarchitecture is described in HIPERLAN Type 2, Data Link Control (DLC)Layer; Part 4, Extension for Home Environment, ETSI TS 101 761-4 V1.2.1(2000-12). A subset of wireless LANs is Wireless Personal Area Networks(PANs), of which the Bluetooth Standard is the best known. The BluetoothSpecial Interest Group, Specification Of The Bluetooth System, Version1.1, Feb. 22, 2001, describes the principles of Bluetooth deviceoperation and communication protocols.

[0010] The IEEE 802.11 Wireless LAN Standard defines at least twodifferent physical (PHY) specifications and one common medium accesscontrol (MAC) specification. The IEEE 802.11(a) Standard is designed tooperate in unlicensed portions of the radio spectrum, usually either inthe 2.4 GHz Industrial, Scientific, and Medical (ISM) band or the 5 GHzUnlicensed-National Information Infrastructure (U-NII) band. It usesorthogonal frequency division multiplexing (OFDM) to deliver up to 54Mbps data rates. The IEEE 802.11 (b) Standard is designed for the 2.4GHz ISM band and uses direct sequence spread spectrum (DSSS) to deliverup to 11 Mbps data rates. The IEEE 802.11 Wireless LAN Standarddescribes two major components, the mobile station and the fixed accesspoint (AP). IEEE 802.11 networks can also have an independentconfiguration where the mobile stations communicate directly with oneanother, without support from a fixed access point.

[0011] A single cell wireless LAN using the IEEE 802.11 Wireless LANStandard is an Independent Basic Service Set (IBSS) network. An IBSS hasan optional backbone network and consists of at least two wirelessstations. A multiple cell wireless LAN using the IEEE 802.11 WirelessLAN Standard is an Extended Service Set (ESS) network. An ESS satisfiesthe needs of large coverage networks of arbitrary size and complexity.

[0012] Each wireless station and access point in an IEEE 802.11 wirelessLAN implements the MAC layer service, which provides the capability forwireless stations to exchange MAC frames. The MAC frame transmitsmanagement, control, or data between wireless stations and accesspoints. After a station forms the applicable MAC frame, the frame's bitsare passed to the Physical Layer for transmission.

[0013] Before transmitting a frame, the MAC layer must first gain accessto the network. Three interframe space (IFS) intervals defer an IEEE802.11 station's access to the medium and provide various levels ofpriority. Each interval defines the duration between the end of the lastsymbol of the previous frame, to the beginning of the first symbol ofthe next frame. The Short Interframe Space (SIFS) provides the highestpriority level by allowing some frames to access the medium beforeothers, such as an Acknowledgement (ACK) frame, a Clear to Send (CTS)frame, or a subsequent fragment burst of a previous data frame. Theseframes require expedited access to the network to minimize frameretransmissions.

[0014] The Priority Interframe Space (PIFS) is used for high priorityaccess to the medium during the contention-free period. The pointcoordinator in the access point connected to backbone network, controlsthe priority-based Point Coordination Function (PCF) to dictate whichstations in cell can gain access to the medium. The point coordinator inthe access point sends a contention-free poll frame to a station,granting the station permission to transmit a single frame to anydestination. All other stations in the cell can only transmit duringcontention-free period if the point coordinator grants them access tothe medium. The end of the contention-free period is signaled by thecontention-free end frame sent by the point coordinator, which occurswhen time expires or when the point coordinator has no further frames totransmit and no stations to poll.

[0015] The distributed coordination function (DCF) Interframe Space(DIFS) is used for transmitting low priority data frames during thecontention-based period. The DIFS spacing delays the transmission oflower priority frames to occur later than the priority-basedtransmission frames. An Extended Interframe Space (EIFS) goes beyond thetime of a DIFS interval, as a waiting period when a bad receptionoccurs. The EIFS interval provides enough time for the receiving stationto send an acknowledgment (ACK) frame.

[0016] During the contention-based period, the distributed coordinationfunction (DCF) uses the Carrier-Sense Multiple Access With CollisionAvoidance (CSMA/CA) contention-based protocol, which is similar to IEEE802.3 Ethernet. The CSMA/CA protocol minimizes the chance of collisionsbetween stations sharing the medium, by waiting a random backoffinterval, if the station's sensing mechanism indicates a busy medium.The period of time immediately following traffic on the medium is whenthe highest probability of collisions occurs, especially where there ishigh utilization. Once the medium is idle, CSMA/CA protocol causes eachstation to delay its transmission by a random backoff time, therebyminimizing the chance it will collide with those from other stations.

[0017] The CSMA/CA protocol computes the random backoff time as theproduct of a constant, the slot time, times a pseudo-random number RNwhich has a range of values from zero to a collision window CW. Thevalue of the collision window for the first try to access the network isCW1, which yields the first try random backoff time. If the first try toaccess the network by a station fails, then the CSMA/CA protocolcomputes a new CW by doubling the current value of CW as CW2=CW1 times2. The value of the collision window for the second try to access thenetwork is CW2, which yields the second try random backoff time. Thisprocess by the CSMA/CA protocol of increasing the delay beforetransmission is called binary exponential backoff. The reason forincreasing CW is to minimize collisions and maximize throughput for bothlow and high network utilization. Stations with low network utilizationare not forced to wait very long before transmitting their frame. On thefirst or second attempt, a station will make a successful transmission.However, if the utilization of the network is high, the CSMA/CA protocoldelays stations for longer periods to avoid the chance of multiplestations transmitting at the same time. If the second try to access thenetwork fails, then the CSMA/CA protocol computes a new CW by againdoubling the current value of CW as CW3=CW1 times 4. The value of thecollision window for the third try to access the network is CW3, whichyields the third try random backoff time. The value of CW increases torelatively high values after successive retransmissions, under hightraffic loads. This provides greater transmission spacing betweenstations waiting to transmit.

[0018] Collision Avoidance Techniques

[0019] Four general collision avoidance approaches have emerged: [1]Carrier Sense Multiple Access (CSMA) [see, F. Tobagi and L. Kleinrock,“Packet Switching in Radio Channels: Part I—Carrier Sense MultipleAccess Models and their Throughput Delay Characteristics”, IEEETransactions on Communications, Vol 23, No 12, Pages 1400-1416, 1975],[2] Multiple Access Collision Avoidance (MACA) [see, P. Karn, “MACA—ANew Channel Access Protocol for Wireless Ad-Hoc Networks”, Proceedingsof the ARRL/CRRL Amateur Radio Ninth Computer Networking Conference,Pages 134-140, 1990.], [3] their combination CSMA/CA, and [4] collisionavoidance tree expansion.

[0020] CSMA allows access attempts after sensing the channel foractivity. Still, simultaneous transmit attempts lead to collisions, thusrendering the protocol unstable at high traffic loads. The protocol alsosuffers from the hidden terminal problem.

[0021] The latter problem was resolved by the MACA protocol, whichinvolves a three-way handshake [P. Karn, supra]. The origin node sends arequest to send (RTS) notice of the impending transmission. A responseis returned by the destination if the RTS notice is receivedsuccessfully and the origin node proceeds with the transmission. Thisprotocol also reduces the average delay as collisions are detected upontransmission of merely a short message, the RTS. With the length of thepacket included in the RTS and echoed in the clear to send (CTS)messages, hidden terminals can avoid colliding with the transmittedmessage. However, this prevents the back-to-back re-transmission in caseof unsuccessfully transmitted packets. A five-way handshake MACAprotocol provides notification to competing sources of the successfultermination of the transmission. [see, V. Bharghavan, A. Demers, S.Shenker, and L. Zhang, “MACAW: A media access protocol for wirelessLANs, SIGCOMM '94, Pages 212-225, ACM, 1994.]

[0022] CSMA and MACA are combined in CSMA/CA, which is MACA with carriersensing, to give better performance at high loads. A four-way handshakeis employed in the basic contention-based access protocol used in theDistributed Coordination Function (DCF) of the IEEE 802.11 Standard forWireless LANs. [see, IEEE Standards Department, D3, “Wireless MediumAccess Control and Physical Layer WG,” IEEE Draft Standard P802.11Wireless LAN, January 1996.]

[0023] Collisions can be avoided by splitting the contending terminalsbefore transmission is attempted. In the pseudo-Bayesian control method,each terminal determines whether it has permission to transmit using arandom number generator and a permission probability “p” that depends onthe estimated backlog. [see, R. L. Rivest, “Network control by BayesianBroadcast”, IEEE Trans. Inform. Theory, Vol IT 25, pp. 505-515,September 1979]

[0024] To resolve collisions, subsequent transmission attempts aretypically staggered randomly in time using the following two approaches:binary tree and binary exponential backoff.

[0025] Upon collision, the binary tree method requires the contendingnodes to self-partition into two groups with specified probabilities.This process is repeated with each new collision. The order in whichcontending nodes transmit is determined either by serial or parallelresolution of the tree. [see, J. L. Massey, “Collision-resolutionalgorithms and random-access communications”, in Multi-UserCommunication Systems, G. Longo (ed.), CISM Courses and Lectures No.265.New York: Springer 1982, pp.73-137.]

[0026] In the binary exponential backoff approach, a backoff countertracks the number of pauses and hence the number of completedtransmissions before a node with pending packets attempts to seize thechannel. A contending node initializes its backoff counter by drawing arandom value, given the backoff window size. Each time the channel isfound idle, the backoff counter is decreased and transmission isattempted upon expiration of the backoff counter. The window size isdoubled every time a collision occurs, and the backoff countdown startsagain. [see, A. Tanenbaum, Computer Networks, 3^(rd) ed., Upper SaddleRiver, N.J., Prentice Hall, 1996] The Distributed Coordination Function(DCF) of the IEEE 802.11 Standard for Wireless LANs employs a variant ofthis contention resolution scheme, a truncated binary exponentialbackoff, starting at a specified window and allowing up to a maximumbackoff range below which transmission is attempted. [IEEE StandardsDepartment, D3, supra] Different backoff counters may be maintained by acontending node for traffic to specific destinations. [Bharghavan,supra]

[0027] In the IEEE 802.11 Standard, the channel is shared by acentralized access protocol, the Point Coordination Function (PCF),which provides contention-free transfer based on a polling schemecontrolled by the access point (AP) of a basic service set (BSS). [IEEEStandards Department, D3, supra] The centralized access protocol gainscontrol of the channel and maintains control for the entirecontention-free period by waiting a shorter time between transmissionsthan the stations using the Distributed Coordination Function (DCF)access procedure. Following the end of the contention-free period, theDCF access procedure begins, with each station contending for accessusing the CSMA/CA method.

[0028] The 802.11 MAC Layer provides both contention and contention-freeaccess to the shared wireless medium. The MAC Layer uses various MACframe types to implement its functions of MAC management, control, anddata transmission. Each station and access point on an 802.11 wirelessLAN implements the MAC Layer service, which enables stations to exchangepackets. The results of sensing the channel to determine whether themedium is busy or idle, are sent to the MAC coordination function of thestation. The MAC coordination also carries out a virtual carrier senseprotocol based on reservation information found in the Duration Field ofall frames. This information announces to all other stations, thesending station's impending use of the medium. The MAC coordinationmonitors the Duration Field in all MAC frames and places thisinformation in the station's Network Allocation Vector (NAV) if thevalue is greater than the current NAV value. The NAV operates similarlyto a timer, starting with a value equal to the Duration Field of thelast frame transmission sensed on the medium, and counting down to zero.After the NAV reaches zero, the station can transmit, if its physicalsensing of the channel indicates a clear channel.

[0029] At the beginning of a contention-free period, the access pointsenses the medium, and if it is idle, it sends a Beacon packet to allstations. The Beacon packet contains the length of the contention-freeinterval. The MAC coordination in each member station places the lengthof the contention-free interval in the station's Network AllocationVector (NAV), which prevents the station from taking control of themedium until the end of the contention-free period. During thecontention-free period, the access point can send a polling message to amember station, enabling it to send a data packet to any other stationin the BSS wireless cell.

[0030] Quality Of Service (QoS)

[0031] Quality of service (QoS) is a measure of service quality providedto a customer. The primary measures of QoS are message loss, messagedelay, and network availability. Voice and video applications have themost rigorous delay and loss requirements. Interactive data applicationssuch as Web browsing have less restrained delay and loss requirements,but they are sensitive to errors. Non-real-time applications such asfile transfer, Email, and data backup operate acceptably across a widerange of loss rates and delay. Some applications require a minimumamount of capacity to operate at all, for example, voice and video. Manynetwork providers guarantee specific QoS and capacity levels through theuse of Service-Level Agreements (SLAs). An SLA is a contract between anenterprise user and a network provider that specifies the capacity to beprovided between points in the network that must be delivered with aspecified QoS. If the network provider fails to meet the terms of theSLA, then the user may be entitled a refund. The SLA is typicallyoffered by network providers for private line, frame relay, ATM, orInternet networks employed by enterprises.

[0032] The transmission of time-sensitive and data application trafficover a packet network imposes requirements on the delay or delay jitter,and the error rates realized; these parameters are referred togenerically as the QoS (Quality of Service) parameters. Prioritizedpacket scheduling, preferential packet dropping, and bandwidthallocation are among the techniques available at the various nodes ofthe network, including access points, that enable packets from differentapplications to be treated differently, helping achieve the differentquality of service objectives. Such techniques exist in centralized anddistributed variations. The concern herein is with distributedmechanisms for multiple access in cellular packet networks or wirelessad hoc networks.

[0033] Management of contention for the shared transmission medium mustreflect the goals sought for the performance of the overall system. Forinstance, one such goal would be the maximization of goodput (the amountof good data transmitted as a fraction of the channel capacity) for theentire system, or of the utilization efficiency of the RF spectrum;another is the minimization of the worst-case delay. As multiple typesof traffic with different performance requirements are combined intopacket streams that compete for the same transmission medium, amulti-objective optimization is required.

[0034] Ideally, one would want a multiple access protocol that iscapable of effecting packet transmission scheduling as close to theoptimal scheduling as possible, but with distributed control.Distributed control implies both some knowledge of the attributes of thecompeting packet sources and limited control mechanisms.

[0035] To apply any scheduling algorithm in random multiple access, amechanism must exist that imposes an order in which packets will seizethe medium. For distributed control, this ordering must be achievedindependently, without any prompting or coordination from a controlnode. Only if there is a reasonable likelihood that packet transmissionswill be ordered according to the scheduling algorithm, can one expectthat the algorithm's proclaimed objective will be attained.

[0036] The above cited, copending patent application by MathildeBenveniste, entitled “Tiered Contention Multiple Access (TCMA): A Methodfor Priority-Based Shared Channel Access”, describes the TieredContention Multiple Access (TCMA) distributed medium access protocolthat schedules transmission of different types of traffic based on theirQoS service quality specifications. This protocol makes changes to thecontention window following the transmission of a frame, and thereforeis also called Extended-DCF (E-DCF). During the contention window, thevarious stations on the network contend for access to the network. Toavoid collisions, the MAC protocol requires that each station first waitfor a randomly-chosen time period, called an arbitration time. Sincethis period is chosen at random by each station, there is lesslikelihood of collisions between stations. TCMA uses the contentionwindow to give higher priority to some stations than to others.Assigning a short contention window to those stations that should havehigher priority ensures that in most cases, the higher-priority stationswill be able to transmit ahead of the lower-priority stations. TCMAschedules transmission of different types of traffic based on their QoSservice quality specifications. As seen in FIG. 1, which depicts thetiered contention mechanism, a station cannot engage in backoffcountdown until the completion of an idle period of length equal to itsarbitration time.

[0037] The above cited, copending patent application by MathildeBenveniste also applies TCMA to the use of the wireless access point asa traffic director. This application of the TCMA protocol is called thehybrid coordination function (HCF). In HCF, the access point uses apolling technique as the traffic control mechanism. The access pointsends polling packets to a succession of stations on the network. Theindividual stations can reply to the poll with a packet that containsnot only the response, but also any data that needs to be transmitted.Each station must wait to be polled. The access point establishes apolling priority based on the QoS priority of each station.

[0038] What is needed in the prior art is a way to apply the hybridcoordination function (HCF) to wireless cells that have overlappingaccess points contending for the same medium.

SUMMARY OF THE INVENTION

[0039] In accordance with the invention, the Tiered Contention MultipleAccess (TCMA) protocol is applied to wireless cells which haveoverlapping access points contending for the same medium. Quality ofService (QoS) support is provided to overlapping access points toschedule transmission of different types of traffic based on the servicequality specifications of the access points.

[0040] The inventive method reduces interference in a medium betweenoverlapping wireless LAN cells, each cell including an access pointstation and a plurality of member stations. In accordance with theinvention, the method assigns to a first access point station in a firstwireless LAN cell, a first scheduling tag. The scheduling tag has avalue that determines an accessing order for the cell in a transmissionframe, with respect to the accessing order of other wireless cells. Thescheduling tag value is deterministically set. The scheduling tag valuecan be permanently assigned to the access point by its manufacturer, itcan be assigned by the network administrator at network startup, it canbe assigned by a global processor that coordinates a plurality ofwireless cells over a backbone network, it can be drawn from a pool ofpossible tag values during an initial handshake negotiation with otherwireless stations, or it can be cyclically permuted in real-time, on aframe-by-frame basis, from a pool of possible values, coordinating thatcyclic permutation with that of other access points in other wirelesscells.

[0041] An access point station in a wireless cell signals the beginningof an intra-cell contention-free period for member stations in its cellby transmitting a beacon packet. The duration of the intra-cellcontention-free period is deterministically set. The member stations inthe cell store the intra-cell contention-free period value as a NetworkAllocation Vector (NAV). Each member station in the cell decrements thevalue of the NAV in a manner similar to other backoff time values,during which it will delay accessing the medium.

[0042] In accordance with the invention, the method assigns to the firstaccess point station, a first inter-cell contention-free period value,which gives notice to any other cell receiving the beacon packet, thatthe first cell has seized the medium for the period of time representedby the value. The inter-cell contention-free period value isdeterministically set. Further in accordance with the invention, anystation receiving the beacon packet immediately broadcasts acontention-free time response (CFTR) packet containing a copy of thefirst inter-cell contention-free period value. In this manner, thenotice is distributed to a second access point station in anoverlapping, second cell. The second access point stores the firstinter-cell contention-free period value as an Inter-BSS NetworkAllocation Vector (IBNAV). The second access point decrements the valueof IBNAV in a manner similar to other backoff time values, during whichit will delay accessing the medium.

[0043] Still further in accordance with the invention, the method alsoassigns to first member stations in the first cell, a first shorterbackoff value for high Quality of Service (QoS) data and a first longerbackoff value for lower QoS data. The backoff time is the interval thata member station waits after the expiration of the contention-freeperiod, before the member station contends for access to the medium.Since more than one member station in a cell may be competing foraccess, the actual backoff time for a particular station can be selectedas one of several possible values. In one embodiment, the actual backofftime for each particular station is deterministically set, so as toreduce the length of idle periods. In another embodiment, the actualbackoff time for each particular station is randomly drawn from a rangeof possible values between a minimum delay interval to a maximum delayinterval. The range of possible backoff time values is a contentionwindow. The backoff values assigned to a cell may be in the form of aspecified contention window. High QoS data is typically isochronous datasuch as streaming video or audio data that must arrive at itsdestination at regular intervals. Low QoS data is typically filetransfer data and email, which can be delayed in its delivery and yetstill be acceptable. The Tiered Contention Multiple Access (TCMA)protocol coordinates the transmission of packets within a cell, so as togive preference to high QoS data over low QoS data, to insure that therequired quality of service is maintained for each type of data.

[0044] The method similarly assigns to a second access point station ina second wireless LAN cell that overlaps the first sell, a secondcontention-free period value longer than the first contention-freeperiod value. The method also assigns to second member stations in thesecond cell, a second shorter backoff value for high QoS data and asecond longer backoff value for lower QoS data. The first and secondcells are considered to be overlapped when one or more stations in thefirst cell inadvertently receive packets from member stations or theaccess point of the other cell. The invention reduces the interferencebetween the overlapped cells by coordinating the timing of theirrespective transmissions, while maintaining the TCMA protocol'spreference for the transmission of high QoS data over low QoS data ineach respective cell.

[0045] During the operation of two overlapped cells, the methodtransmits a first beacon packet including the intra-cell contention-freeperiod value (the increment to the NAV) and inter-cell contention-freeperiod value (the CFTR), from the first access point to the first memberstations in the first cell. The beacon packet is received by the memberstations of the first cell and can be inadvertently received by at leastone overlapped member station of the second cell. Each member station inthe first cell increments its NAV with the intra-cell contention-freeperiod value and stores the inter-cell contention-free period value asthe CFTR.

[0046] In accordance with the invention, each station that receives thefirst beacon packet, immediately responds by transmitting a firstcontention-free time response (CFTR) packet that contains a copy of theinter-cell contention-free period value (CFTR). A CFTR packet istransmitted from the first member stations in the first cell and also bythe overlapped member stations of the second cell. The effect of thetransmission of CFTR packets from member stations in the second cell isto alert the second access point and the second member stations in thesecond cell, that the medium has been seized by the first access pointin the first cell. When the second access point in the second cellreceives the CFTR packet it stores the a copy of the inter-cellcontention-free period value as the IBNAV.

[0047] Similar to a station's Network Allocation Vector (NAV), a firstIBNAV is set at the second access point to indicate the time the mediumwill be free again. Also similar to the NAV, the first IBNAV isdecremented with each succeeding slot, similar to the decrementing ofother backoff times. When the second access point receives the firstIBNAV representing the first cell's contention-free period value, thesecond access point must respect the first IBNAV value and delaytransmitting its beacon packet and the exchange of other packets in thesecond cell until the expiration of the received, first IBNAV.

[0048] When the second access point has decremented the first IBNAV tozero, the second access point transmits its second beacon packetincluding its second contention-free period values of NAV and a secondIBNAV, to the second member stations in the second cell. Each stationthat receives the second beacon packet immediately responds bytransmitting a second contention-free time response (CFTR) packet thatcontains a copy of the second IBNAV inter-cell contention-free periodvalue. The second CFTR packet is transmitted from the second memberstations in the second cell and also by the overlapped member stationsof the first cell. The effect of the transmission of the second CFTRpackets from overlapped member stations in the first cell is to alertthe first access point and the first member stations in the first cell,that the medium has been seized by the second access point in the secondcell. When the first access point in the first cell receives the CFTRpacket it stores the a copy of the second IBNAV inter-cellcontention-free period value, to indicate the time the medium will befree again. The second IBNAV is decremented with each succeeding frame,similar to the decrementing of other backoff times.

[0049] The second member stations in the second cell wait for completionof the count down of their NAVs to begin the TCMA protocol of countingdown the second shorter backoff for high QoS data and then transmittingsecond high QoS data packets.

[0050] Meanwhile, the first access point in the first cell waits forcompletion of the count down of the second IBNAV inter-cellcontention-free period before starting the countdown of its own NAV forits own intra-cell contention-free period. The first member stations inthe first cell wait for the count down of their NAVs, to begin the TCMAprotocol of counting down the first longer backoff for low QoS data andthen transmitting first low QoS data.

[0051] Meanwhile the second member stations are waiting for the TCMAprotocol of counting down the second longer backoff for lower QoS databefore transmitting the second lower QoS data.

[0052] In this manner, interference in a medium between overlappingwireless LAN cells is reduced.

[0053] Potential collisions between cells engaged in centralized accesscan be averted or resolved by the TCMA protocol. In accordance with theinvention, deterministically set backoff delays are used, which tend toreduce the length of the idle periods. The possibility of coincident oroverlapping contention-free periods between neighboring cells iseliminated through the use of an “interference sensing” method employinga new frame.

[0054] The invention enables communication of channel occupancyinformation to neighboring access points. When a beacon packet istransmitted, and before transmission of any other data or pollingpackets, all stations hearing the beacon will respond by sending aframe, the contention-free time response (CFTR), that will contain theduration of the contention-free period found in the beacon. An accesspoint in neighboring cells, or stations attempting contention-basedchannel access, which receive this message from a station in the celloverlapping region, are thus alerted that the channel has been seized byan access point. Similar to a station's Network Allocation Vector (NAV),an Inter-Cell Network Allocation Vector at the access point accordinglyindicates when the time the channel will be free again. Unless theInter-Cell Network Allocation Vector is reset, the access point willdecrease its backoff value only after the expiration of the Inter-CellNetwork Allocation Vector, according to the backoff countdown rules.

[0055] In another aspect of the invention, potential collisions betweendifferent access points engaged in centralized access can be averted orresolved by using deterministic backoff delays, which avoid collisionsbetween access points, and eliminate gaps between consecutivepoll/response exchanges or contention-free bursts (CFBs) between theaccess point and its associated stations.

[0056] The resulting invention applies the Tiered Contention MultipleAccess (TCMA) protocol to wireless cells which have overlapping accesspoints contending for the same medium.

DESCRIPTION OF THE FIGURES

[0057]FIG. 1 depicts the tiered contention mechanism.

[0058]FIGS. 1A through 1J show the interaction of two wireless LAN cellswhich have overlapping access points contending for the same medium, inaccordance with the invention.

[0059]FIG. 1K shows a timing diagram for the interaction of two wirelessLAN cells in FIGS. 1A through 1J, in accordance with the invention.

[0060]FIG. 1L shows the IEEE 802.11 packet structure for a beaconpacket, including the increment to the NAV period and the CFTR period,in accordance with the invention.

[0061]FIG. 1M shows the IEEE 802.11 packet structure for a CFTR packet,including the CFTR period, in accordance with the invention.

[0062]FIG. 2 illustrates the ordering of transmissions from three groupsof BSSs.

[0063]FIG. 3 illustrates how three interfering BSSs share the samechannel for two consecutive frames.

[0064]FIG. 4 illustrates how three interfering BSSs, each with two typesof traffic of different priorities, share the same channel in twoconsecutive frames.

[0065]FIG. 5 illustrates the possible re-use of tags.

[0066]FIG. 6 illustrates the deterministic post-backoff.

[0067]FIG. 7 shows the relationships of repeating sequences of CFBs.

[0068]FIG. 8 illustrates the role of pegging in a sequence of CFBs bythree overlapping access points.

[0069]FIG. 9 illustrates the start-up procedure for a new access point,HC2, given an existing access point, HC1.

[0070]FIG. 10 shows the relationship of repeating sequences of Tier ICFBs.

[0071]FIG. 11 illustrates the start-up procedure for a new access point,HC2, given an existing access point, HC1.

DISCUSSION OF THE PREFERRED EMBODIMENT

[0072] The invention disclosed broadly relates to telecommunicationsmethods and more particularly relates to Quality-of-Service (QoS)management in multiple access packet networks. Several protocols, eithercentralized or distributed can co-exist on the same channel through theTiered Contention Multiple Access method. The proper arbitration time tobe assigned to the centralized access protocol must satisfy thefollowing requirements: (i) the centralized access protocol enjoys toppriority access, (ii) once the centralized protocol seizes the channel,it maintains control until the contention-free period ends, (iii) theprotocols are backward compatible, and (iv) Overlapping Basic ServiceSets (OBSSs) engaged in centralized-protocol can share the channelefficiently.

[0073] In accordance with the invention, the Tiered Contention MultipleAccess (TCMA) protocol is applied to wireless cells which haveoverlapping access points contending for the same medium. Quality ofService (QoS) support is provided to overlapping access points toschedule transmission of different types of traffic based on the servicequality specifications of the access points.

[0074] The inventive method reduces interference in a medium betweenoverlapping wireless LAN cells, each cell including an access pointstation and a plurality of member stations. FIGS. 1A through 1J show theinteraction of two wireless LAN cells which have overlapping accesspoints contending for the same medium, in accordance with the invention.The method assigns to a first access point station in a first wirelessLAN cell, a first scheduling tag. The scheduling tag has a value thatdetermines an accessing order for the cell in a transmission frame, withrespect to the accessing order of other wireless cells. The schedulingtag value is deterministically set. The scheduling tag value can bepermanently assigned to the access point by its manufacturer, it can beassigned by the network administrator at network startup, it can beassigned by a global processor that coordinates a plurality of wirelesscells over a backbone network, it can be drawn from a pool of possibletag values during an initial handshake negotiation with other wirelessstations, or it can be cyclically permuted in real-time, on aframe-by-frame basis, from a pool of possible values, coordinating thatcyclic permutation with that of other access points in other wirelesscells.

[0075] The interaction of the two wireless LAN cells 100 and 150 inFIGS. 1A through 1J is shown in the timing diagram of FIG. 1K. Thetiming diagram of FIG. 1K begins at instant T0, goes to instant T9, andincludes periods P1 through P8, as shown in the figure. The variouspackets discussed below are also shown in FIG. 1K, placed at theirrespective times of occurrence. An access point station in a wirelesscell signals the beginning of an intra-cell contention-free period formember stations in its cell by transmitting a beacon packet. FIG 1Ashows access point 152 of cell 150 connected to backbone network 160,transmitting the beacon packet 124. In accordance with the invention,the beacon packet 124 includes two contention-free period values, thefirst is the Network Allocation Vector (NAV) (or alternately itsincremental value ΔNAV), which specifies a period value P3 for theintra-cell contention-free period for member stations in its own cell.Member stations within the cell 150 must wait for the period P3 beforebeginning the Tiered Contention Multiple Access (TCMA) procedure, asshown in FIG. 1K. The other contention-free period value included in thebeacon packet 124 is the Inter-BSS Network Allocation Vector (IBNAV),which specifies the contention-free time response (CFTR) period P4. Thecontention-free time response (CFTR) period P4 gives notice to any othercell receiving the beacon packet, such as cell 100, that the first cell150 has seized the medium for the period of time represented by thevalue P4.

[0076] The beacon packet 124 is received by the member stations 154A(with a low QoS requirement 164A) and 154B (with a high QoS requirement164B) in the cell 150 during the period from T1 to T2. The memberstations 154A and 154B store the value of ΔNAV=P3 and begin countingdown that value during the contention free period of the cell 150. Theduration of the intra-cell contention-free period ΔNAV=P3 isdeterministically set. The member stations in the cell store theintra-cell contention-free period value P3 as the Network AllocationVector (NAV). Each member station in the cell 150 decrements the valueof the NAV in a manner similar to other backoff time values, duringwhich it will delay accessing the medium. FIG. 1L shows the IEEE 802.11packet structure 260 for the beacon packet 124 or 120, including theincrement to the NAV period and the CFTR period. The beacon packetstructure 260 includes fields 261 to 267. Field 267 specifies the ΔNAVvalue of P3 and the CFTR value of P4. In accordance with the invention,the method assigns to the first access point station, a first inter-cellcontention-free period value, which gives notice to any other cellreceiving the beacon packet, that the first cell has seized the mediumfor the period of time represented by the value. The inter-cellcontention-free period value is deterministically set.

[0077] Further in accordance with the invention, any station receivingthe beacon packet 124 immediately rebroadcasts a contention-free timeresponse (CFTR) packet 126 containing a copy of the first inter-cellcontention-free period value P4. The value P4 specifies the Inter-BSSNetwork Allocation Vector (IBNAV), i.e., the contention-free timeresponse (CFTR) period that the second access point 102 must wait, whilethe first cell 150 has seized the medium. FIG. 1B shows overlap station106 in the region of overlap 170 transmitting the CFTR packet 126 tostations in both cells 100 and 150 during the period from T1 to T2. FIG.1M shows the IEEE 802.11 packet structure 360 for a CFTR packet 126 or122, including the CFTR period. The CFTR packet structure 360 includesfields 361 to 367. Field 367 specifies the CFTR value of P4. In thismanner, the notice is distributed to the second access point station 102in the overlapping, second cell 100.

[0078]FIG. 1C shows the point coordinator in access point 152 of cell150 controlling the contention-free period within the cell 150 by usingthe polling packet 128 during the period from T2 to T3. In the meantime, the second access point 102 in the second cell 100 connected tobackbone network 110, stores the first inter-cell contention-free periodvalue P4 received in the CFTR packet 126, which it stores as theInter-BSS Network Allocation Vector (IBNAV). The second access point 102decrements the value of IBNAV in a manner similar to other backoff timevalues, during which it will delay accessing the medium.

[0079] Still further in accordance with the invention, the method usesthe Tiered Contention Multiple Access (TCMA) protocol to assign to firstmember stations in the first cell 150, a first shorter backoff value forhigh Quality of Service (QoS) data and a first longer backoff value forlower QoS data. FIG. 1D shows the station 154B in the cell 150, having ahigh QoS requirement 164B, decreasing its High QoS backoff period tozero and beginning TCMA contention to transmit its high QoS data packet130 during the period from T3 to T4. The backoff time is the intervalthat a member station waits after the expiration of the contention-freeperiod P3, before the member station 154B contends for access to themedium. Since more than one member station in a cell may be competingfor access, the actual backoff time for a particular station can beselected as one of several possible values. In one embodiment, theactual backoff time for each particular station is deterministicallyset, so as to reduce the length of idle periods. In another embodiment,the actual backoff time for each particular station is randomly drawnfrom a range of possible values between a minimum delay interval to amaximum delay interval. The range of possible backoff time values is acontention window. The backoff values assigned to a cell may be in theform of a specified contention window. High QoS data is typicallyisochronous data such as streaming video or audio data that must arriveat its destination at regular intervals. Low QoS data is typically filetransfer data and email, which can be delayed in its delivery and yetstill be acceptable. The Tiered Contention Multiple Access (TCMA)protocol coordinates the transmission of packets within a cell, so as togive preference to high QoS data over low QoS data, to insure that therequired quality of service is maintained for each type of data.

[0080] The method similarly assigns to the second access point 102station in the second wireless LAN cell 100 that overlaps the first sell150, a second contention-free period value CFTR=P7 longer than the firstcontention-free period value CFTR=P4. FIG. 1E shows the second accesspoint 102 in the cell 100 transmitting its beacon packet 120 includingits contention-free period values of NAV (P6) and IBNAV (P7), to themember stations 104A (with a low QoS requirement 114A), 104B (with ahigh QoS requirement 114B) and 106 in the cell 100 during the periodfrom T4 to T5. FIG. 1F shows that each station, including the overlapstation 106, that receives the second beacon packet 120, immediatelyresponds by retransmitting a second contention-free time response (CFTR)packet 122 that contains a copy of the second inter-cell contention-freeperiod value P7 during the period from T4 to T5.

[0081]FIG. 1G shows the point coordinator in access point 102 of cell100 controlling the contention-free period within cell 100 using thepolling packet 132 during the period from T5 to T6.

[0082] The method uses the Tiered Contention Multiple Access (TCMA)protocol to assign to second member stations in the second cell 100, asecond shorter backoff value for high QoS data and a second longerbackoff value for lower QoS data. FIG. 1H shows the station 104B in thecell 100, having a high QoS requirement 114B, decreasing its High QoSbackoff period to zero and beginning TCMA contention to transmit itshigh QoS data packet 134 during the period from T6 to T7. FIG. 1I showsthe first member stations 154A and 154B in the first cell 150 waitingfor the count down of their NAVs, to begin the TCMA protocol of countingdown the first longer backoff for low QoS data and then transmittingfirst low QoS data 136 during the period from T7 to T8. FIG. 1J showsthe second member stations 104A, 104B, and 106 are waiting for the TCMAprotocol of counting down the second longer backoff for lower QoS databefore transmitting the second lower QoS data 138 during the period fromT8 to T9.

[0083] The first and second cells are considered to be overlapped whenone or more stations in the first cell can inadvertently receive packetsfrom member stations or the access point of the other cell. Theinvention reduces the interference between the overlapped cells bycoordinating the timing of their respective transmissions, whilemaintaining the TCMA protocol's preference for the transmission of highQoS data over low QoS data in each respective cell.

[0084] During the operation of two overlapped cells, the method in FIG.1A transmits a first beacon packet 124 including the intra-cellcontention-free period value (the increment to the NAV) and inter-cellcontention-free period value (the CFTR), from the first access point 152to the first member stations 154B and 154A in the first cell 150. Thebeacon packet is received by the member stations of the first cell andinadvertently by at least one overlapped member station 106 of thesecond cell 100. Each member station 154B and 154A in the first cellincrements its NAV with the intra-cell contention-free period value P3and stores the inter-cell contention-free period value P4 as the CFTR.

[0085] In accordance with the invention, each station that receives thefirst beacon packet 124, immediately responds by transmitting a firstcontention-free time response (CFTR) packet 126 in FIG. 1B that containsa copy of the inter-cell contention-free period P4 value (CFTR). A CFTRpacket 126 is transmitted from the first member stations 154B and 154Ain the first cell 150 and also by the overlapped member stations 106 ofthe second cell 100. The effect of the transmission of CFTR packets 126from member stations 106 in the second cell 100 is to alert the secondaccess point 102 and the second member stations 104A and 104B in thesecond cell 100, that the medium has been seized by the first accesspoint 152 in the first cell 150. When the second access point 102 in thesecond cell 100 receives the CFTR packet 126 it stores the a copy of theinter-cell contention-free period value P4 as the IBNAV.

[0086] Similar to a station's Network Allocation Vector (NAV), an IBNAVis set at the access point to indicate the time the medium will be freeagain. Also similar to the NAV, the IBNAV is decremented with eachsucceeding slot, similar to the decrementing of other backoff times.When the second access point receives a new IBNAV representing the firstcell's contention-free period value, then the second access point mustrespect the IBNAV value and delay transmitting its beacon packet and theexchange of other packets in the second cell until the expiration of thereceived, IBNAV.

[0087] Later, as shown in FIG. 1E, when the second access point 102transmits its second beacon packet 120 including its secondcontention-free period values of NAV (P6) and IBNAV (P7), to the secondmember stations 104A, 104B and 106 in the second cell 100, each stationthat receives the second beacon packet, immediately responds bytransmitting a second contention-free time response (CFTR) packet 122 inFIG. 1F, that contains a copy of the second inter-cell contention-freeperiod value P7. A CFTR packet 122 is transmitted from the second memberstations 104A, 104B and overlapped station 106 in the second cell andalso by the overlapped member stations of the first cell. The effect ofthe transmission of CFTR packets from overlapped member station 106 isto alert the first access point 152 and the first member stations 145Aand 154B in the first cell 150, that the medium has been seized by thesecond access point 102 in the second cell 100. When the first accesspoint 152 in the first cell 150 receives the CFTR packet 122 it storesthe a copy of the second inter-cell contention-free period value P7 asan IBNAV, to indicate the time the medium will be free again. The IBNAVis decremented with each succeeding slot, similar to the decrementing ofother backoff times.

[0088] The second member stations 104A, 104B, and 106 in the second cell100 wait for completion of the count down of their NAVs to begin theTCMA protocol of counting down the second shorter backoff for high QoSdata and then transmitting second high QoS data packets, as shown inFIGS. 1G and 1H.

[0089] Meanwhile, the first access point 152 in the first cell 150 waitsfor completion of the count down of the second inter-cellcontention-free period P7 in its IBNAV in FIGS. 1G and 1H beforestarting the countdown of its own NAV for its own intra-cellcontention-free period. The first member stations 154A and 154B in thefirst cell 150 wait for the count down of their NAVs, to begin the TCMAprotocol of counting down the first longer backoff for low QoS data andthen transmitting first low QoS data in FIG. 1.

[0090] Meanwhile the second member stations 104A, 104B, and 106 arewaiting for the TCMA protocol of counting down the second longer backofffor lower QoS data before transmitting the second lower QoS data 138 inFIG. 1J.

[0091] In this manner, interference in a medium between overlappingwireless LAN cells is reduced.

DETAILED DESCRIPTION OF THE INVENTION

[0092] TCMA can accommodate co-existing Extended DistributedCoordination Function (E-DCF) and centralized access protocols. In orderto ensure that the centralized access protocol operating under HybridCoordination Function (HCF) is assigned top priority access, it musthave the shortest arbitration time. Its arbitration time is determinedby considering two additional requirements: uninterrupted control of thechannel for the duration of the contention-free period, and backwardcompatibility.

[0093] Uninterrupted Contention-Free Channel Control

[0094] The channel must remain under the control of the centralizedaccess protocol until the contention-free period is complete once it hasbeen seized by the centralized access protocol. For this, it issufficient that the maximum spacing between consecutive transmissionsexchanged in the centralized access protocol, referred to as the centralcoordination time (CCT), be shorter than the time the channel must beidle before a station attempts a contention-based transmission followingthe end of a busy-channel time interval. The centralized access protocolhas a CCT equal to the Priority Interframe Space (PIFS). Hence, nostation may access the channel by contention, using either thedistributed coordination function (DCF) or Extended-DCF (E-DCF) accessprocedure, before an idle period of length of the DCF Interframe Space(DIFS) equaling PIFS+1(slot time) following the end of a busy-channeltime interval. This requirement is met by DCF. For E-DCF, it would besufficient for the Urgency Arbitration Time (UAT) of a class j, UAT_(j),to be greater than PIFS for all classes j>1.

[0095] Backward Compatibility

[0096] Backward compatibility relates to the priority treatment oftraffic handled by enhanced stations (ESTAs) as compared to legacystations (STAs). In addition to traffic class differentiation, the ESTAsmust provide certain traffic classes with higher or equal priorityaccess than that provided by the STAs. That means that certain trafficclasses should be assigned a shorter arbitration times than DIFS, the defacto arbitration time of legacy stations.

[0097] Because the time in which the “clear channel assessment” (CCA)function can be completed is set at the minimum attainable for the IEEE802.11 physical layer (PHY) specification, the arbitration times of anytwo classes of different priority would have to be separated by at leastone “time slot”. This requirement implies that the highest prioritytraffic class would be required to have an arbitration time equal toDIFS−1(slot time)=PIFS.

[0098] Though an arbitration time of PIFS appears to fail meeting therequirement for uninterrupted control of the channel during thecontention-free period, it is possible for an ESTA to access the channelby E-DCF using an arbitration time of PIFS and, at the same time, allowpriority access to the centralized access protocol at PIFS. This isachieved as follows. Contention-based transmission is restricted tooccur after a DIFS idle period following the end of a busy channelperiod by ensuring that the backoff value of such stations is drawn froma random distribution with lower bound that is at least 1. Given thatall backlogged stations resume backoff countdown after a busy-channelinterval with a residual backoff of at least 1, an ESTA will attempttransmission following completion of the busy interval only after anidle period equal to PIFS+1(slot time)=DIFS. This enables thecentralized access protocol to maintain control of the channel withoutcolliding with contention-based transmissions by ESTAs attempting toaccess the channel using E-DCF.

[0099] To see that the residual backoff value of a backlogged stationwill be greater than or equal to 1 whenever countdown is resumed at theend of a busy channel period, consider a station with a backoff valuem>0. The station will decrease its residual backoff value by 1 followingeach time slot during which the channel remains idle. If m reaches 0before countdown is interrupted by a transmission, the station willattempt transmission. The transmission will either fail, leading to anew backoff being drawn, or succeed. Therefore, countdown will beresumed after the busy-channel period ends, only with a residual backoffof 1 or greater. Consequently, if the smallest random backoff that canbe drawn is 1 or greater, an ESTA will always wait for at least a DIFSidle interval following a busy period before it attempts transmission.

[0100] Only one class can be derived with priority above legacy throughdifferentiation by arbitration time alone, by using the arbitration timeof PIFS. Multiple classes with that priority can be obtained bydifferentiation through other parameters, such as the parameters of thebackoff time distribution; e.g. the contention window size. For all theclasses so derived, a DIFS idle period will follow a busy channelinterval before the ESTA seizes the channel if the restriction isimposed that the backoff value of such stations be drawn from a randomdistribution with lower bound of at least 1.

[0101] Because PIFS is shorter than DIFS, the traffic classes witharbitration time equal to PIFS will have higher access priority than thetraffic classes with arbitration time equal to DIFS. As seen in FIG. 1,which depicts the tiered contention mechanism, a station cannot engagein backoff countdown until the completion of an idle period of lengthequal to its arbitration time. Therefore, a legacy station will beunable to resume backoff countdown at the end of a busy-channelinterval, if an ESTA with arbitration time of PIFS has a residualbackoff of 1. Moreover, a legacy station will be unable to transmituntil all higher-priority ESTAs with residual backoff of 1 havetransmitted. Only legacy stations that draw a backoff value of 0 willtransmit after a DIFS idle period, thus competing for the channel withthe higher priority stations. This occurs only with a probability lessthan 3 per cent, since the probability of drawing a random backoff of 0from the range [0,31] is equal to {fraction (1/32)}.

[0102] Top Priority for the Centralized Access Protocol

[0103] For the centralized access protocol to enjoy the highest priorityaccess, it must have an arbitration time shorter than PIFS by at least atime slot; that is, its arbitration time must equal PIFS−1(slottime)=the Short Interframe Space (SIFS). As in the case of the highesttraffic priority classes for ESTAs accessing the channel by E-DCF, therandom backoff values for the beacon of the centralized access protocolmust be drawn from a range with a lower bound of at least 1. Using thesame reasoning as above, the centralized access protocol will nottransmit before an idle period less than PIFS=SIFS+1 (slot time), thusrespecting the inter-frame spacing requirement for a SIFS idle periodwithin frame exchange sequences. Consequently, the shorter arbitrationtime assigned to the centralized access protocol ensures that itaccesses the channel with higher priority than any station attemptingcontention-based access through E-DCF, while at the same time respectingthe SIFS spacing requirement.

[0104] It should be noted that while collisions are prevented betweenframe exchanges during the contention-free period, collisions arepossible both between the beacons of centralized access protocols ofdifferent BSSs located within interfering range [having coverageoverlap], and between the beacon of a centralized access protocol andstations accessing the channel by contention using E-DCF. Theprobability of such collisions is low because higher priority nodes withresidual backoff value m equal to 1 always seize the channel beforelower priority nodes. Inter-access point collisions are resolved throughthe backoff procedure of TCMA.

[0105] Inter-access Point Contention

[0106] Potential collisions between BSSs engaged in centralized accesscan be averted or resolved by a backoff procedure. The complicationarising here is that a random backoff delay could result in idle periodslonger periods than the SIFS+1(slot time)=PIFS, which is what ensurespriority access to the centralized protocol over E-DCF trafficcontention-based traffic. Hence, the collisions with contention-basedtraffic would occur. Using short backoff windows in order to avoid thisproblem would increase the collisions experienced. In accordance withthe invention, deterministically set backoff delays are used, which tendto reduce the length of the idle periods.

[0107] Another aspect of inter-BSS interference that affects theperformance of centralized protocols adversely is the possibleinterruption with a collision of what starts as an interference-freepoll/response exchange between the access point and its associatedstations. The possibility of coincident or overlapping contention-freeperiods between neighboring BSSs is eliminated through the use of an“interference sensing” method employing a new frame.

[0108] Deterministic Backoff Procedure for the Centralized AccessProtocol

[0109] A modified backoff procedure is pursued for the beacons of thecentralized access protocols. A backoff counter is employed in the sameway as in TCMA. But while the backoff delay in TCMA is selected randomlyfrom a contention window, in the case of the centralized access protocolbeacons, the backoff value is set deterministically.

[0110] Scheduling of packet transmission occurs once per frame, at thebeginning of the frame. Only the packets queued at the start of a framewill be transmitted in that frame. It is assumed that BSSs aresynchronized. A means for achieving such synchronization is through theexchange of messages relayed by boundary stations [stations in theoverlapping regions of neighboring BSSs].

[0111] The backoff delay is selected through a mechanism called “tagscheduling”. Tags, which are ordinal labels, are assigned to differentBSSs. BSSs that do not interfere with one another may be assigned thesame tag, while BSSs with the potential to interfere with one anothermust receive different tags. For each frame, the tags are ordered in away that is known a priori. This order represents the sequence in whichthe BSS with a given tag will access the channel in that frame. Thebackoff delay increases with the rank of the “tag” that has beenassigned to the BSS for the current frame, as tags are permuted to giveeach group of BSS with the same tag a fair chance at the channel. Forinstance, a cyclic permutation for three tags, t=1, 2, 3, would give thefollowing ordering: 1, 2, 3 for the first frame, 3, 1, 2 next, and then2, 3, 1. One could also use other permutation mechanisms that areadaptive to traffic conditions and traffic priorities. The difference inthe backoff delays corresponding to two consecutive tags is one timeslot. FIG. 2 illustrates the ordering of transmissions from three groupsof BSSs.

[0112] A backoff counter is associated with each backoff delay. It isdecreased according to the rules of TCMA using the arbitration time ofShort Interframe Space (SIFS) as described in the preceding section.That is, once the channel is idle for a time interval equal to SIFS, thebackoff counter associated with the centralized protocol of the BSS isdecreased by 1 for each slot time the channel is idle. Access attemptoccurs when the backoff counter expires. The minimum backoff valueassociated with the highest-ranking tag is 1. FIG. 3 illustrates howthree interfering BSSs share the same channel for two consecutiveframes. The tags assigned in each of the two frames are (1, 2), (2, 3),and (3, 1) for the three BSSs, respectively. The backoff delays for thethree tags are 1, 2, and 3 time slots.

[0113] When the channel is seized by the centralized protocol of a BSS,it engages in the polling and transmission functions for a timeinterval, known as the contention-free period. Once the channel has beensuccessfully accessed that way, protection by the Network AllocationVector (NAV) prevents interference from contention based trafficoriginating within that BSS. Avoidance of interference from neighboringBSS is discussed below. A maximum limit is imposed on the reservationlength in order to even out the load on the channel from different BSSsand allow sufficient channel time for contention-based traffic.

[0114] It is important to note the advantage of using deterministicbackoff delays, versus random. Assuming an efficient (i.e., compact) tagre-use plan, deterministic backoff delays increase the likelihood that abeacon will occur precisely after an idle period of length SIFS+1=PIFS.This will enable the centralized protocol to gain access to the channel,as a higher priority class should, before contention-based traffic canaccess the channel at DIFS=PIFS+1. Using a random backoff delay insteadmight impose a longer idle period and hence, give rise to collisionswith contention-based traffic. Use of short backoff windows to avoidthis problem would be ill advised, since that would result in collisionbetween the various BSS beacons.

[0115] Though the backoff delays are set in a deterministic manner,there are no guarantees that collisions will always be avoided. Unlessthe duration of the contention-free period is the same for all BSSs,there is the possibility that interfering BSSs will attempt to accessthe channel at once. In case of such a collision, the backoff procedurestarts again with the backoff delay associated with the tag assigned tothe BSS, decreased by 1, and can be repeated until expiration of theframe. At the start of a new frame, a new tag is assigned to the BSSaccording to the pre-specified sequence, and the deferral time intervalassociated with the new tag is used.

[0116] Collisions are also possible if tag assignments are imperfect(interfering BSSs are assigned the same tag). In the event of such acollision, transmission should be retried with random backoff. In orderto deal with either type of collision, resolution occurs by drawing arandom delay from a contention window size that increases with thedeterministic backoff delay associated with the tag in that frame.Though random backoff is used in this event, starting with deterministicbackoff helps reduce contention time.

[0117] In a hybrid scenario, random backoff can be combined with tagscheduling. Instead of using backoff delays linked to the rank of a tagin a frame, the contention window size from which the backoff delay isdrawn would increase with decreasing rank. The advantage of such anapproach is to relax the restrictions on re-use by allowing thepossibility that potentially interfering stations will be assigned thesame tag. The disadvantage is that the Inter-BSS Contention Period(IBCP) time needed to eliminate contention by E-DCF traffic increases.

[0118] Interference Sensing

[0119] Interference sensing is the mechanism by which the occupancystatus of a channel is determined. The access point only needs to knowof channel activity in interfering BSSs. The best interference sensingmechanism is one that ensures that the channel is not usedsimultaneously by potentially interfering users. This involves listeningto the channel by both the access point and stations. If the accesspoint alone checks whether the channel is idle, the result does notconvey adequate information on the potential for interference at areceiving station, nor does it address the problem of interferencecaused to others by the transmission, as an access point may not be ableto hear transmissions from its neighboring access points, yet there ispotential of interference to stations on the boundary of neighboringBSSs. Stations must detect neighboring BSS beacons and forward theinformation to their associated access point. However, transmission ofthis information by a station would cause interference within theneighboring BSS.

[0120] In order to enable communication of channel occupancy informationto neighboring access points, the invention includes the followingmechanism. When a beacon packet is transmitted, and before transmissionof any other data or polling packets, all stations hearing the beaconwill respond by sending a frame, the contention-free time response(CFTR), that will contain the duration of the contention-free periodfound in the beacon. An access point in neighboring BSSs, or stationsattempting contention-based channel access, that receive this messagefrom a station in the BSS overlapping region are thus alerted that thechannel has been seized by a BSS. Similar to a station's NetworkAllocation Vector (NAV), an Inter-Cell Network Allocation Vector, alsoreferred to herein as an inter-BSS NAV (IBNAV), is set at the accesspoint, accordingly, indicating the time the channel will be free again.Unless the IBNAV is reset, the access point will decrease its backoffvalue only after the expiration of the IBNAV, according to the backoffcountdown rules.

[0121] Alternatively, if beacons are sent at fixed time increments,receipt of the contention-free time response (CFTR) frame would sufficeto extend the IBNAV. The alternative would be convenient in order toobviate the need for full decoding of the CFTR frame. It is necessary,however, that the frame type of CFTR be recognizable.

[0122] Contention by E-DCF traffic while various interfering BSSsattempt to initiate their contention-free period can be lessened byadjusting the session length used to update the NAV and IBNAV. Thecontention-free period length is increased by a period Inter-BSSContention Period (IBCP) during which the access points only willattempt access of the channel using the backoff procedure, while ESTAswait for its expiration before attempting transmission. This mechanismcan reduce the contention seen by the centralized protocols whenemploying either type of backoff delay, random or deterministic. Withdeterministic backoff delays, IBCP is set equal to the longest residualbackoff delay possible, which is T(slot time), where T is the number ofdifferent tags. Given reasonable re-use of the tags, the channel timedevoted to the IBCP would be less with deterministic backoff delays, ascompared to the random.

[0123] QoS Management

[0124] A QoS-capable centralized protocol will have traffic withdifferent time delay requirements queued in different priority buffers.Delay-sensitive traffic will be sent first, followed by traffic withlower priority. Tag scheduling is used again, but now there are two ormore backoff values associated with each tag, a shorter value for thehigher priority traffic and longer ones for lower priority. A BSS willtransmit its top priority packets first, as described before. Once thetop priority traffic has been transmitted, there would be further delaybefore the BSS would attempt to transmit lower priority traffic in orderto give neighboring BSSs a chance to transmit their top prioritypackets. As long as any of the deferral time intervals for low-prioritytraffic is longer than the deferral time intervals for higher prioritytraffic of any tag, in general all neighboring BSSs would have a chanceto transmit all pending top-priority packets before any lower-prioritypackets are transmitted.

[0125]FIG. 4 illustrates how three interfering BSSs, each with two typesof traffic of different priorities, share the same channel in twoconsecutive frames. As before, the tags assigned in each of the twoframes are (1, 2), (2, 3), and (3, 1) for the three BSSs, respectively.The deferral times for the top priority traffic are 1, 2, and 3 timeslots for tags 1, 2, and 3, respectively. The deferral times for thehigher priority traffic are 4, 5, and 6 time slots for tags 1, 2, and 3,respectively.

[0126] Tag Assignments

[0127] A requirement in assigning tags to BSS is that distinct tags mustbe given to user entities with potential to interfere. This is not adifficult requirement to meet. In the absence of any information, adifferent tag could be assigned to each user entity. In that case,non-interfering cells will use the channel simultaneously even thoughthey have different tags. Interference sensing will enable reuse of thechannel by non-interfering BSSs that have been assigned different tags.

[0128] There are advantages, however, in reducing the number ofdifferent tags. For instance, if the interference relationships betweenuser entities are known, it is advantageous to assign the same tag tonon-interfering BSS, and thus have a smaller number of tags. Theutilization of bandwidth, and hence total throughput, would be greateras shorter deferral time intervals leave more of the frame timeavailable for transmission. Moreover, an efficient (i.e., compact) tagre-use plan will decrease the likelihood of contention between thecentralized protocol beacons of interfering BSSs contenting for accessand E-DCF traffic. This problem is mitigated by using the IBCP time inthe IBNAV, but re-use will reduce the length of this time.

[0129] The assignment of tags to cells can be done without knowledge ofthe location of the access points and/or the stations. Tag assignment,like channel selection can be done at the time of installation. Andagain, like dynamic channel selection, it can be selected by the accesspoint dynamically. RF planning, which processes signal-strengthmeasurements can establish re-use groups and thus reduce the requirednumber of tags. FIG. 5, which includes FIGS. 5(a) and 5(b), illustratesthe possible re-use of tags. In FIG. 5(a), the access points are locatedat ideal spots on a hexagonal grid to achieve a regular tessellatingpattern. In FIG. 5(b), the access points have been placed as convenientand tags are assigned to avoid overlap. Imperfect tag assignments willlead to collisions between the access points, but such collisions can beresolved.

[0130] To recap, arbitration times have been assigned to a centralizedaccess protocol that co-exists with ESTAs accessing the channel throughE-DCF. The centralized access protocol has the top priority, while E-DCFcan offer traffic classes with priority access both above and below thatprovided by legacy stations using DCF.

[0131] Table 1 illustrates the parameter specification for K+1 differentclasses according to the requirements given above. The centralizedaccess protocol is assigned the highest priority classification, andhence the shortest arbitration time. The top k−1 traffic classes for theE-DCF have priority above legacy but below the centralized accessprotocol; they achieve differentiation through the variation of thecontention window size as well as other parameters. E-DCF trafficclasses with priority above legacy have a lower bound, rLower, of thedistribution from which backoff values are drawn that is equal to 1 orgreater. Differentiation for classes with priority below legacy isachieved by increasing arbitration times; the lower bound of the randombackoff distribution can be 0.

[0132] BSSs within interfering range of one another compete for thechannel through a deterministic backoff procedure employing tagscheduling, which rotates the backoff value for fairness amongpotentially interfering BSS. Re-use of a tag is permitted innon-interfering BSS. Multiple queues with their own backoff valuesenable prioritization of different QoS traffic classes.

[0133] Contention-free Bursts

[0134] In accordance with the invention, potential collisions betweendifferent BSSs engaged in centralized access can be averted/resolved bydetermtinistic backoff delays, which avoid collisions between accesspoints, and eliminate gaps between consecutive poll/response exchangesbetween the access point and its associated stations. These are referredto as contention-free bursts (CFBs).

[0135] Deterministic Backoff Procedure for the Centralized AccessProtocol

[0136] A modified backoff procedure is pursued for the beacons of thecentralized access protocols. A backoff counter is employed in the sameway as in TCMA. But while the backoff delay in TCMA is selected randomlyfrom a contention window, in the case of the centralized access protocolbeacons, the backoff value is set deterministically to a fixed valueBkoff, at the end of its contention-free session. Post-backoff is turnedon.

[0137] The backoff counter is decreased according to the rules of TCMAusing the arbitration time AIFS=SIFS as described in the precedingsection. That is, once the channel is idle for a time interval equal toSIFS, the backoff counter associated with the centralized protocol ofthe BSS is decreased by 1 for each slot time the channel is idle. Accessattempt occurs when the backoff counter expires. An HC will restart itsbackoff after completing its transmission. The deterministicpost-backoff procedure is illustrated in FIG. 6

[0138] When the channel is seized by the centralized protocol of a BSS,it engages in the polling and transmission functions for a timeinterval, known as the contention-free period. Once the channel has beensuccessfully accessed that way, protection by the NAV preventsinterference from contention based traffic originating in the BSS.Avoidance of interference from neighboring BSS is discussed below.

[0139] Non-conflicting Contiguous Sequences of CFBs

[0140] As long as the value of Bkoff is greater than or equal to themaximum number of interfering BSS, it is possible for thecontention-free periods of a cluster of neighboring/overlapping BSSs torepeat in the same order without a collision between them. CFBs ofdifferent BSSs can be made to follow one another in a contiguoussequence, thus maximizing access of the centralized protocol to thechannel. This can be seen as follows.

[0141] Given a sequence of successful CFBs initiated by different BSSs,subsequent CFBs will not conflict because the follower's backoff counteralways exceeds that of the leader by at least 1. If the previous CFBswere contiguous (that is, if consecutive CFBs were separated by idlegaps of length PIFS, the new CFBs will be also continuous because thefollower's backoff delay exceeds that of the leader by exactly 1.Channel access attempts by E-DCF stations require an idle gap of lengthequal to DIFS or greater. FIG. 7 shows the relationships of repeatingsequences of CFBs.

[0142] In order to maintain contiguity, an HC that does not have anytraffic to transmit when its backoff expires, it will transmit a shortpacket—a “peg”—and then engage in post-backoff. This way no gaps oflength DIFS+1 are left idle until all HCs have completed one CFB percycle, and restarted the backoff countdown procedure. E-DCF stations arethus prevented from seizing the channel until each BSS completes atleast one CFB per cycle. FIG. 8 illustrates the role of pegging in asequence of CFBs by three overlapping access points.

[0143] Finally it is shown how such a contiguous sequence canconstructed by analyzing how a new access point initiates its first CFB.Every time a new access point is installed, it must find its position inthe repeating sequence of CFBs. The new access point listens to thechannel for the desired cycle, trying to recognize the sequence. Itlistens for an “idle” PIFS following a busy channel. When that occurs,or after counting Bkoff time slots, whichever comes first, the newaccess point starts looking for the first idle longer than PIFS, whichsignifies the end of the sequence of CFBs. As long as the Bkoff isgreater than the number of interfering BSS, there will always be such anidle period. The access point sets its post-backoff delay so that ittransmits always right at the end of the CFB sequence. That is, if attime t, an idle>PIFS has been detected, the access point's backoff attime t is Bkoff−x(t), where x(t) is the number of idle time slots afterPIFS. FIG. 9 illustrates this start-up procedure for a new access point,HC2, given an existing access point, HC1.

[0144] Interference Sensing

[0145] Interference sensing is the mechanism by which the occupancystatus of a channel is determined. The access point only needs to knowof channel activity in interfering BSSs. The best interference sensingmechanism is one that ensures that the channel is not usedsimultaneously by potentially interfering users. This involves listeningto the channel by both the access point and stations. If the accesspoint alone checks whether the channel is idle, the result does notconvey adequate information on the potential for interference at areceiving station, nor does it address the problem of interferencecaused to others by the transmission, as an access point may not be ableto hear transmissions from its neighboring access points, yet there ispotential of interference to stations on the boundary of neighboringBSS. Stations must detect neighboring BSS beacons and forward theinformation to their associated access point. However, transmission ofthis information by a station would cause interference within theneighboring BSS.

[0146] In order to enable communication of channel occupancy informationto neighboring access points, the following mechanism is proposed. Whena beacon packet is transmitted, and before transmission of any otherdata or polling packets, all stations not associated with the accesspoint that hear the beacon will respond by sending a frame, thecontention-free time response (CFTR), that will contain the duration ofthe contention-free period found in the beacon. An associated stationwill transmit the remaining duration of the contention-free period whenpolled. An access point in neighboring BSSs, or stations attemptingcontention-based channel access, that receive this message from astation in the BSS overlapping region are thus be alerted that thechannel has been seized by a BSS. Similar to a station's NAV, aninter-BSS NAV (IBNAV) will be set at the access point accordinglyindicating the time the channel will be free again. Unless the IBNAV isreset, the access point will decrease its backoff value only after theexpiration of the IBNAV, according to the backoff countdown rules.

[0147] Alternatively, if beacons are sent at fixed time increments,receipt of the CFTR frame would suffice to extend the IBNAV. Thealternative would be convenient in order to obviate the need for fulldecoding of the CFTR frame. It is necessary, however, that the frametype of CFTR be recognizable.

[0148] Contention by E-DCF traffic while various interfering BSSsattempt to initiate their contention-free period can be lessened byadjusting the session length used to update the NAV and IBNAV. Thecontention-free period length is increased by a period IBCP (inter-BSScontention period) during which the access points only will attemptaccess of the channel using the backoff procedure, while ESTAs wait forits expiration before attempting transmission. This mechanism can reducethe contention seen by the centralized protocols when employing eithertype of backoff delay—random or deterministic.

[0149] QoS Management

[0150] A QoS-capable centralized protocol will have traffic withdifferent time delay requirements queued in different priority buffers.Delay-sensitive traffic will be sent first, followed by traffic withlower priority. A BSS will schedule transmissions from separate queuesso that the QoS requirements are met. It will transmit its top prioritypackets first, as described before. Once the top priority traffic hasbeen transmitted, the BSS would attempt to transmit lower prioritytraffic in the CFBs allotted.

[0151] Three parameters are employed to help manage QoS. Thedeterministic backoff delay, Bkoff, and the maximum length of a CFB andof a DCF transmission. Since these parameters determine the relativeallocation of the channel time between the centralized and distributedprotocols, they can be adjusted to reflect the distribution of thetraffic load between the two protocols. It must be kept in mind,however, that the same value of Bkoff should be used by all interferingBSSs.

[0152] QoS Guarantees

[0153] To enable high priority traffic to be delivered within guaranteedlatency limits, a variation of the above method is described. CFBs of anaccess point are separated into two types, or tiers. The first containstime sensitive data and is sent when the period TXdt expires. The secondtier contains time non-sensitive traffic and is sent when the backoffcounter expires as a result of the countdown procedure. When allneighboring BSS have a chance to transmit their time sensitive traffic,the channel is available for additional transmissions before needing totransmit time-sensitive traffic again. Lower priority contention-freedata can be then transmitted, using a backoff-based procedure.

[0154] Tier II CFBs can be initiated in various methods. Two will bedescribed here. They are: (1) random post-backoff, and (2) deterministicpost-backoff. Both methods use the same AIFS used for top-priority EDCFtransmissions, in order to avoid conflict with Tier I CFBs (i.e. anAIFS=PIFS). Conflict with top priority EDCF transmissions can bemitigated in case (1) or prevented in case (2) through the use of theIBNAV with an IBCP.

[0155] Random post-backoff assigns an access point a backoff drawn froma prespecified contention window. A short contention window would leadto conflicts between Tier II CFBs. A long contention window reduces theconflict between interfering BSS attempting to access the channel atonce. Long backoff values would reduce the fraction of the time thechannel carries CFBs. Furthermore, the gaps created by multipleconsecutive idle slots make room for DCF transmissions, reducing furtherthe channel time available to CFBs. A long IBCP value would alleviatesome of the conflict with DCF transmissions.

[0156] Deterministic post-backoff eliminates the problems present withrandom post-backoff. Conflicts with top priority EDCF transmissions canbe prevented with an IBCP of 1. Moreover, as explained above, the TierII CFBs generated by this method, do not conflict with one another andform contiguous repeating sequences.

[0157] Non-conflicting Contiguous Sequences of Tier I CFBs

[0158] Periodic transmission is achieved by maintaining a timer which isreset at the desired period TXdt as soon as the timer expires. A CFB isinitiated upon expiration of the timer. As long as Tier Icontention-free periods are all made the same size (by adding timenon-critical traffic), which is not less than the maximum DCFtransmission or Tier II CFB length, it is possible for thecontention-free periods of a cluster of neighboring/overlapping BSSs torepeat in the same order without a collision between them. CFBs ofdifferent BSSs can be made to follow one another in a contiguoussequence, thus maximizing access of the centralized protocol to thechannel. This can be seen as follows.

[0159] Given a sequence of successful CFBs initiated by different BSSs,subsequent CFBs will not conflict because their timers will expire atleast TICFBLength apart. If the leading access point's timer expireswhile the channel is busy, it will be able to start a new CFB before thefollower HC because DCF transmissions are of equal or shorter length,and Type II CFBs have equal or shorter length.

[0160] If the previous CFBs were contiguous (that is, if consecutiveCFBs were separated by idle gaps of length PIFS), the new CFBs will bealso continuous because the follower's timer will expire on or beforethe completion of the leader's CFB because their CFBs have the samelength. Channel access attempts by E-DCF stations or Tier II CFBsrequire an idle gap of length equal to DIFS or greater, and hence theycannot be interjected. FIG. 10 shows the relationship of repeatingsequences of Tier I CFBs.

[0161] Finally it is shown how such a contiguous sequence canconstructed by analyzing how a new access point initiates its first TierI CFB. Every time a new access point is installed, it must find itsposition in the repeating sequence of CFBs. The new access point listensto the channel for the desired cycle, trying to recognize the sequence.It listens for an “idle” PIFS following a busy channel. When thatoccurs, or after a period TXdt, whichever comes first, the new accesspoint starts looking for the first idle longer than PIFS, whichsignifies the end of the sequence of Tier I CFBs. As long as the TXdt isgreater than the number of interfering BSS times the duration of a TierI CFB, TICFBLength, there will always be such an idle period. The accesspoint sets its timer so that it transmits always right at the end of theCFB sequence. That is, if at time t, an idle of length X(t)>PIFS hasbeen detected, the access point's timer at time t is TXdt−X(t)+PIFS.FIG. 11 illustrates this start-up procedure for a new access point, HC2,given an existing access point, HC1.

[0162] Possibility of Collisions

[0163] Though the backoff delays are set in a deterministic manner,there are no guarantees that collisions will always be avoided. Unlessall access points sense the start and end of CFBs at the same time,there is the possibility that interfering BSSs will attempt to accessthe channel at once. This situation arises when there is significantdistance between access points, but not sufficient to eliminateinterference between them. Such a situation can be alleviated throughthe assignment for different channels.

[0164] Arbitration times are assigned to a centralized access protocolthat co-exists with ESTAs accessing the channel through E-DCF. Thecentralized access protocol has the top priority, while E-DCF can offertraffic classes with priority access both above and below that providedby legacy stations using DCF.

[0165] Table 1 illustrates the parameter specification for K+1 differentclasses according to the requirements given above. The centralizedaccess protocol is assigned the highest priority classification, andhence the shortest arbitration time. The top k−1 traffic classes for theE-DCF have priority above legacy but below the centralized accessprotocol; they achieve differentiation through the variation of thecontention window size as well as other parameters. E-DCF trafficclasses with priority above legacy have a lower bound, rLower, of thedistribution from which backoff values are drawn that is equal to 1 orgreater Differentiation for classes with priority below legacy isachieved by increasing arbitration times; the lower bound of the randombackoff distribution can be 0. TABLE 1 TCMA Priority Class DescriptionPriority Class Description Arbitration time rLower 0 Centralized accessprotocol SIFS > = 1 CFBs 1 to E-DCF Traffic with priority PIFS = SIFS +1 (slot > = 1 k − 1 above Legacy or Centralized time) access protocolTier II CFBs k E-DCF Legacy-equivalent DIFS = SIFS + 2 (slot 0 trafficpriority time) n = k + E-DCF Traffic priority below > DIFS = SIFS + 0 1to K Legacy (2 + n − k) (slot time)

[0166] BSSs within short interfering range of one another can competefor and share the channel through the use of a deterministic backoffprocedure employing post-backoff. Contiguous repeating sequences ofcontention-free periods provide the centralized protocol efficientaccess to the channel which is shared by E-DCF transmissions. Therelative channel time allotted to the two protocols can be adjusted bytuning parameters of the protocol. Scheduling of traffic queued inmultiple queues at the access point can meet QoS requirements. Morestringent latency requirements can be met with a two-tiered method,which employs both a timer and post-back of to initiate CFBs.

[0167] CFB contiguity is preserved when using deterministic post-backoffor if CFBs of constant length are used whenever transmission is causedby the expiration of the TXdt timer—the Tier I approach. Contiguity isnot necessarily preserved, however, if the CFBs have variable lengthwhen the Tier I approach is used. Any gaps that would arise in this casewould allow contention-based transmissions to be interjected, thusrisking delays and possible collisions between HCs.

[0168] Because of the fixed CFB length requirement, whereas the Tier Iapproach delivers regularly-spaced CFBs, using it alone, without a TierII protocol, results in inefficient utilization of the channel. The samefixed bandwidth allocation to each BSS gives rise to situations wherechannel time allocated for a CFB to one BSS may be left idle whileanother BSS is overloaded. The Tier II protocols provide for dynamicbandwidth allocation among BSSs.

[0169] Various illustrative examples of the invention have beendescribed in detail. In addition, however, many modifications andchanges can be made to these examples without departing from the natureand spirit of the invention.

1. A method for enabling a plurality of overlapped wireless LAN cells to have contention-free access to a medium, each cell including a respective plurality of member stations, comprising: assigning a first inter-cell contention-free period value to a first access point station in a first cell, associated with an accessing order in the medium for member stations in first and second cells of the plurality of cells during a transmission frame; transmitting by the first access point station in the first cell, a beacon packet containing the first inter-cell contention-free period value; receiving the beacon packet at an overlapped station occupying the first cell and the second cell, and forwarding the inter-cell contention-free period value to member stations in the second cell, to delay transmissions by member stations in the second cell until after said first inter-cell contention-free period; assigning a second inter-cell contention-free period value to a second access point station in the second cell, associated with said accessing order; transmitting by the second access point station in the second cell, a second beacon packet containing the second inter-cell contention-free period value; receiving the second beacon packet at an overlapped station occupying said first cell and said second cell, and forwarding the second inter-cell contention-free period value to member stations in the first cell to delay transmissions by member stations in the first cell until after said second inter-cell contention-free period; receiving the first inter-cell contention-free period value and the second inter-cell contention-free period value at a third access point station in a third cell overlapped with said first cell and said second cell; selecting a backoff time for said third access point station following the second inter-cell contention-free period, to begin transmission by the third access point station in the third cell of a third beacon packet containing a third inter-cell contention-free period value; and receiving the third beacon packet at an overlapped station occupying said first cell and an overlapped station occupying said second cell, and forwarding the third inter-cell contention-free period value to member stations in the first and second cells to delay transmissions by member stations in the first and second cells until after said third inter-cell contention-free period.
 2. The method of claim 1, which further comprises: transmitting in the first beacon packet to member stations in the first cell, an intra-cell contention-free period value, during which they will delay accessing the medium.
 3. The method of claim 2, which further comprises: transmitting a peg packet by the third access point station in the third cell to deter member stations in said overlapped first cell from contending for the medium.
 4. A method for enabling a plurality of overlapped wireless LAN cells to have contention-free access to a medium, each cell including a respective plurality of member stations, comprising: assigning a first inter-cell contention-free period value to a first access point station in a first cell, associated with an accessing order in the medium for member stations in first and second cells of the plurality of cells during a transmission frame; transmitting by the first access point station in the first cell, a beacon packet containing the first inter-cell contention-free period value; assigning a second inter-cell contention-free period value to a second access point station in the second cell, associated with said accessing order; transmitting by the second access point station in the second cell, a second beacon packet containing the second inter-cell contention-free period value; receiving the first inter-cell contention-free period value and the second inter-cell contention-free period value at a third access point station in a third cell overlapped with said first cell and said second cell; selecting a backoff time for said third access point station following the second inter-cell contention-free period, to begin transmission by the third access point station in the third cell of a third beacon packet containing a third inter-cell contention-free period value; and receiving the third inter-cell contention-free period value at member stations in the first and second cells to delay transmissions by member stations in the first and second cells until after said third inter-cell contention-free period.
 5. The method of claim 4, which further comprises: transmitting in the first beacon packet to member stations in the first cell, an intra-cell contention-free period value, during which they will delay accessing the medium.
 6. The method of claim 5, which further comprises: transmitting a peg packet by the third access point station in the third cell to deter member stations in said overlapped first cell from contending for the medium. 